1. Field of the Invention
The present invention relates to a method of forming embedded interconnections of copper on a surface of a substrate such as a semiconductor wafer, and a structure of such embedded interconnections of copper.
2. Description of the Related Art
Generally, aluminum alloys have heretofore been used as the materials of interconnections for use in semiconductor devices. For lower electric resistance and greater migration resistance, however, embedded interconnections of copper produced by a damascene process, and such embedded interconnections of copper arranged in multiple layers are employed.
Conventional multilayer embedded interconnections of copper have suffered various problems. Such problems will be described below with reference to FIGS. 1A through 1C of the accompanying drawings which illustrate a process of successive steps of forming multilayer embedded interconnections of copper.
As shown in FIG. 1A, an interconnection 111 of copper is embedded in the upper surface of an insulating layer 110 of silicon dioxide (SiO2). Another insulating layer 120 of silicon dioxide (SiO2) is disposed on the insulating layer 110 and the interconnections 111 for insulating the interconnections of copper (Cu) 111 in an upper layer. When the insulating layer 120 is deposited on the interconnections layer 111, an exposed upper surface 111a of the interconnections 111 is undesirably oxidized by oxygen.
As shown in FIG. 1B, an etchant (etching gas) is applied to etch the insulating layer 120 through a hole 131 defined in a resist layer pattern 130 on the surface of the insulating layer 120 for thereby forming a hole 121 in the insulating layer 120, which will be filled with a plug for connection to the interconnections 111. When the insulating layer 120 is thus etched, the exposed upper surface 111a of the interconnections 111 is undesirably modified in the composition by the etchant.
As shown in FIG. 1C, when the resist layer 130 (see FIG. 1B) is removed using oxygen, the exposed upper surface 111a of the interconnections 111 is undesirably oxidized by the applied oxygen.
It is therefore an object of the present invention to provide a method of forming embedded interconnections of copper while effectively preventing the exposed surface of an interconnections of copper from being modified or oxidized.
Another object of the present invention is to provide an embedded copper interconnection structure.
According to the present invention, a method of forming embedded interconnections of copper comprises the steps of: forming an insulating layer; forming embedded interconnections of copper in the insulating layer; planarizing (making coplanar) an exposed surface of the insulating layer including an exposed surface of the embedded interconnections of copper; and forming a protective film of silver on the exposed surface of the embedded interconnections of copper. The protective film of silver on the embedded interconnections of copper prevents the embedded interconnections of copper from being oxidized and prevents the surface thereof from being modified.
To form the protective film of silver on the exposed surface of the embedded interconnections of copper, the exposed surface of the embedded interconnections of copper may be plated with silver according to for example, immersion plating. The immersion plating allows the protective film of silver to be selectively formed only on the exposed surface of the embedded interconnections of copper, and also allows the protective film of silver to be formed in a very small thickness. Therefore, the amount of silver in the protective silver film may be relatively small. Since the protective film of silver and the embedded interconnections of copper do not form a solid solution, the electrical resistance of the embedded interconnections remains relatively low even if silver and copper are diffused in each other.
According to the present invention, an embedded copper interconnection structure comprises a substrate, a first insulating layer disposed on the substrate, an embedded interconnection of copper disposed in the first insulating layer, a protective film of silver disposed on the embedded interconnections of copper in the first insulating layer. The structure also includes a second insulating layer disposed on a surface of the first insulating layer including the embedded interconnections of copper having the protective film of silver thereon.